Yes, the USB-C Slim Dock for 24" iMac is compatible with a USB external 2.5 inch hard drive.

• Connect the drive to the USB-A data port (up to 10Gbps) or the USB-C data port (up to 10Gbps).
• Satechi recommends connecting one bus powered device at a time.
• The USB ports are for data transfer and USB peripherals, not for charging.